1. Field of the Invention
The present invention relates to a developer for electrophotography, an image forming apparatus and a process cartridge.
2. Description of the Related Art
Conventionally, in an electrophotographic image forming apparatus or the like, an electrically or magnetically formed latent image is visualized with a toner for electrophotography (may be merely referred to as a “toner” hereinafter). For example, in electrophotography, a latent electrostatic image (latent image) is formed on a photoreceptor, followed by developing the latent image with the toner, to form a toner image. The toner image is typically transferred onto a transfer material such as paper, followed by fixing onto the transfer material. In the fixing image for fixing the toner image on the transfer paper, a thermal fixing system, such as a heating roller fixing system or heating belt fixing system, has been generally and widely used because of its excellent energy efficiency. The electrophotographic developer includes a one-component developer formed of a toner and a two-component developer formed of a toner and a carrier. The former is said to be suitable for small apparatuses which do not have much inner space and the latter is said to be suitable for large apparatuses capable of forming high-quality images such as high-resolution images and color images at high speed because the toner is well charged when stirred with the carrier. However, recently, needs for the two-component developer capable of complying with demands for higher-quality images more easily than the one-component developer have been increasing. Further, in addition to demands for smaller image forming apparatuses, higher speed and higher quality images, demands for energy saving and low-temperature quick fixability have been increasing, and the developer, i.e., the toner and the carrier are exposed to acute stress because of being circulated at high speed in order to comply these. Furthermore, the toner and the carrier are required to have smaller sizes, particularly the toner is required to have higher durability and the developer is required not to give an excessive stress to the toner. In addition, the toner is required to have better storage stability before used, which is inconsistent with the low-temperature quick fixability of the toner.
Namely, recently, there are increasing demands from the market for image forming apparatuses of high speed and energy saving, and therefore a toner having excellent low-temperature fixability and capable of providing high quality images is desired. To achieve the low-temperature fixability of the toner, the softening point of the binder resin contained in the toner needs to be low, but use of the binder resin having a low softening point tends to occur deposition of part of a toner image onto a surface of a fixing member during fixing, which will then be transferred to photocopy paper, which is so-called offset (also referred to as hot offset hereinafter). In addition to this, the heat resistant storage stability of the toner reduces, and therefore toner particles are fused to each other particularly in high temperature environments, which is so called blocking. Other than the above, use of the binder resin having a low softening point causes problems that the toner is fused to an inner area of a developing unit, or to a carrier, and the toner tends to cause filming on a surface of a photoreceptor.
As for the technique for solving the aforementioned problems, it has been known that a crystalline resin is used as a binder resin of the toner. Specifically, the crystalline resin is capable of decreasing the softening point of the toner to around the melting point thereof by sharply softening at the melting point of the resin, while maintaining the heat resistant storage stability at the temperature equal to or lower than the melting point. Accordingly, use of the crystalline resin in the toner realizes both the low-temperature fixability and heat resistant storage stability of the toner.
As for the toner using the crystalline resin, for example, there is disclosed a toner in which a crystalline resin obtained by elongating crystalline polyester with diisocyanate is used as a binder resin (see Japanese published examined application Nos. JP-04-024702-B and JP-04-024703-B). In the case of these proposed toners, the low-temperature fixability of the toner is excellent, but the hot offset resistance of the toner is insufficient, and therefore these toners do not meet the quality required by the market of recent years.
There is disclosed a toner using a crystalline resin which has a crosslink structure due to an unsaturated bond containing sulfonic acid groups (see Japanese Patent No. JP-3910338-B1 (Japanese published unexamined application No. JP-2001-305796-A). This toner improves its hot offset resistance compared to that achieved by the conventional technique. Moreover, there is disclosed a technique related resin particles having excellent low temperature fixing ability and heat resistant storage stability, in which a ratio of a softening point to a peak temperature of heat of melting, and viscoelasticity are specified (see Japanese published unexamined application No. JP-2010-077419-A).
In the case of the aforementioned toners each of which uses the crystalline resin as a main component of the binder resin, however, the toner has excellent shock resistance owing to the characteristics of the crystalline resin, but has insufficient indentation hardness, such as Vickers hardness. Accordingly, there are problems that due to the stress caused by stirring in a developing unit, depositions of the toner to the carrier or inner side of the developing unit and filming onto a photoconductor tend to occur, and deterioration in charging ability and flowability of the toner tends to occur due to buried external additives in the toner particles. Therefore, the carrier needs to give low stress to the toner to stabilize images.
As a carrier giving low stress to a toner, a carrier formed by a polymerization method is disclosed in Japanese published unexamined application No. JP-2005-215397-A.
It is preferable that the carrier typically has low specific gravity to decrease stress to the toner, and a carrier including a dispersed magnetic material has been increasing recently. The carrier including a dispersed magnetic material keeps has a low specific gravity while keeping magnetism because a magnetic powder having strong magnetism is dispersed in a resin having los specific gravity. Japanese published unexamined application No. JP-2005-215397-A discloses this carrier including a dispersed magnetic material and having low specific gravity. Japanese published unexamined application No. JP-8-95308-A discloses a carrier including a dispersed magnetic material, formed of associated a particulate copolymer of an ethylenic unsaturated monomer and a crosslinker, and plural magnetic particulate materials. Japanese published unexamined application No. JP-2008-268489-A discloses a carrier including a mother particle and child particles melted and bonded thereto.
However, they are effective for toner transferability and decreasing stress thereto to some extent, but not enough.
Because of these reasons, a need exist for a developer for electrophotography, including a toner including a crystalline resin in an amount of 50% by weight or more per 100% by weight of a binder resin, as a substantial main component, which produces high-quality images while preventing the toner form contaminating a carrier and inner apparatus and an external additive from being buried in the toner due to insufficient resistance thereof to stress because of the crystalline resin.